"In search for the Soviet ICBM test site, the US intelligence employed its most sophisticated weapon of
the time -- the U-2 high-altitude spy plane. Unreachable for the Soviet interceptors, the U-2s were flying
along major railroads of the Soviet Union trying to pinpoint the new test site. In the summer of 1957, only
weeks after the first test flight of the Soviet ICBM, they discovered the R-7 / Sputnik launch pad."
Reference-  Russian Space Web / Baikonur
The search strategy of flying out railroad lines was clever because the R-7 rocket stages were built in
Northern Russia and had to be transported to the secret testing site in Kazakhstan.
Text on image reads - Baikonur Cosmodrome - At the Sputnik Pad, the teardrop shaped flame
trench stands out as a high flying U-2 spy plane snapped this photo.  U.S. fear brought about
these secret overflights which helped access the Soviet Unions ICBM threat to America.  
The "Launch Pad #1" photo, credit to- USA CIA